At present, a fuel cell system, which includes a fuel cell that generates electric power upon receiving source gases (a fuel gas and an oxidizing gas), has been proposed and put into practical use. When electric power is generated by this fuel cell system, water is generated inside the fuel cell by an electrochemical reaction; however, there is a case in which water is retained in the source gas passage of the fuel cell and blocks the inflow of source gas. Further, when the fuel cell system is to be operated in a low-temperature environment, such as a below-zero environment, there is a case wherein water retained in the electrodes (a catalyst layer and a diffusion layer) of the fuel cell is frozen and sharply deteriorates the starting function.
As prior art that resolves the above described variety of problems that occur due to the water generated inside a fuel cell, a technique (a scavenging technique) has been proposed whereby, when the operation of a fuel cell is halted, dry oxygen or dry hydrogen is supplied to a source gas passage, thereby removing any water remaining in the fuel cell. Furthermore, recently, a technique has been proposed whereby a calculation is performed to determine the balance of the water remaining in a fuel cell, and based on the thus obtained water balance, the flow rate of gas to be supplied to the fuel cell is increased or decreased to adjust the internal moisture state of the fuel cell (see, for example, Japanese Patent Application Laid-Open No. 2004-119052).